In rubber materials for large tires ranging from truck and bus tires to aircraft tires, diene rubbers have conventionally been used as principal materials. Among these diene rubbers, it is the common practice to use natural rubber for the improvement of the heat generation and breaking strength of tires, a styrene-butadiene copolymer rubber for the improvement of the abrasion resistance and cut resistance of tires, and a polybutadiene rubber for the improvement of the abrasion resistance of tires. There have been used rubber compositions with one or more of these rubber materials mixed in a suitable amount according to the purpose of improvement.
In particular, natural rubber is used principally in a rubber for tread parts of large tires because of substantially high loads applied to the tires. A polybutadiene rubber having a high cis content is generally blended further.
Abrasion resistance and crack resistance are both important properties for large tires. They are however contradictory to each other. In a mixture of a natural rubber and a polybutadiene rubber having a high cis content, a greater proportion of the polybutadiene rubber leads to an improvement in abrasion resistance but to a deterioration in crack resistance. While a smaller proportion of the polybutadiene results in insufficient abrasion resistance.
Extensive investigations have therefore been conducted to date on the improvement of polybutadiene rubbers, leading to many proposals. Exemplary proposals include a method in which a polybutadiene rubber having a wide molecular weight distribution is used to improve tack and/or green strength (Japanese Patent Application Laid-Open No. 45337/1984) and a method in which the molecular weight of a polybutadiene rubber is increased to improve abrasion resistance. These methods however are accompanied by the drawback that the processing characteristics are deteriorated although abrasion resistance is improved.
On the other hand, rubber compositions blended with a polybutadiene rubber having a high trans content are dealt with in Japanese Patent Application Laid-Open No. 197749/1985. Primary objects however reside in improvements in processing characteristics and cold flow characteristics but not in improvements in abrasion resistance. As a matter of fact, abrasion resistance is improved very little.